1. Field of the Invention
The present invention relates to a method and apparatus for preventing fluid leakage through a sealing mechanism of a melt pump into a vessel during the period of establishing a vacuum therein and increasing the viscosity of the material fed thereto. More particularly, the method and apparatus of the present invention prevent air leakage up the annular space, defined by an extractor screw shaft and housing, through the shaft packing and around the extractor screw or other melt pump into a reactor during the period of establishing a vacuum therein and increasing the viscosity of the polymer fed thereto.
2. The Prior Art
This invention is particularly adapted for use in a process for the preparation of linear condensation polyesters, e.g., as described in U.S. Pat. No. 3,689,461 to Balint et al., hereby incorporated by reference. In the polycondensation reactors, the polymer melt with monomer and/or solvent dissolved therein is subjected to temperature and pressure conditions favoring utilization of the lower boiling fraction. Elevated temperatures and subatmospheric pressures [e.g., 270.degree.-300.degree. C. and 1 to 5 Torr (133-667 Pa)] are typical. A vertical extractor screw or other melt pump, such as a gear pump, discharges the product horizontally through an exit port above the extractor screw shaft and shaft housing. During normal operations, the extractor screw generates sufficient pressure in the vicinity of the exit port to preclude any fluid leakage up the annular space, defined by the extractor screw shaft and its housing, into the reactor; however, the highly viscous discharge product occasionally leaks down the annular space and may ultimately freeze the shaft, i.e., prevent its rotation. Also, during periods of establishing a vacuum in the reactor and increasing the viscosity of the material fed thereto, e.g., during startup, the pressure generated by the extractor screw is generally insufficient to prevent fluid leakage, usually air, up and into the reactor. If a sufficient amount of air leaks into the reactor, a vacuum cannot be drawn. Further, the air blows polymer onto the reactor walls and down where it stays and degrades. Also, the oxygen present in the air causess degradation products. Thus, it is necessary to maintain adequate seals around the screw shaft.
Prior art techniques for sealing shafts entering or exiting reactors generally involve the use of packed stuffing boxes or of mechanical seals. Stuffing boxes generally have some tendency to leak in vacuum service. Typically, they require considerable attention and frequent tightening and even replacement of worn packing to keep them in satisfactory operating condition. In addition, the polymer may be contaminated by particles of worn packing, by any lubricant which may be necessary or desirable in the stuffing box, and by leakage of air.
Mechanical seals are usually less subject to leakage and wear; they too require occasional replacement and may be subject to lubricant and/or coolant leakage when lubricant or coolant is required. When mechanical seals are used on high viscosity fluids, such as a polymer melt, these fluids tend to cause the sealing means between the rotating shaft and the rotating seal face to become stuck or frozen and this result does not permit the axial movement of the rotating seal face which is necessary for proper operation of a mechanical seal.
The present invention has been developed, therefore, to eliminate the aforementioned fluid leakage. In this regard, U.S. Pat. No. 3,795,386 to Carter et al., hereby incorporated by reference, is believed to be pertinent prior art.